The present invention relates to techniques for obtaining bent glass sheets.
More specifically, the invention relates to those of these techniques in which the glass sheets are made to travel over at least one shaping bed consisting of shaping rods, for example rotating elements arranged along a path having a curved profile in the direction of travel of the glass sheets.
The invention applies for example to the production of automotive glazing, for example of the side window type.
Such bending techniques are currently employed at very high production rates due, in particular, to the possibility of moving the glass sheets along with a spacing of just a few centimeters between them. They allow very good repeatability of the curvature and of the optical quality of the end glazing.
Bending methods and machines are described for example in EP-133 113, EP-133 114 and in international application PCT WO 99/12855. In a known way, the glass sheets are brought to their softening temperature and then they travel in an ascending manner through a shaping bed which has a quarter-circle profile or a profile which is less than a quarter of a circle, in which shaping bed they enter horizontally into a tangential manner and which imparts the desired curvature to the glass sheets.
Once the sheets have been shaped, they are tempered or cooled in order to be hardened, and then a tilting device makes it possible to replace them horizontally, on leaving the shaping bed, on a conveyor carrying them toward a second cooling zone and then toward the exit zone.
This bending method has various disadvantages.
The first is that, because of the limited length of the shaping bed, it is necessary to find a compromise between productivity in respect of which the speed of movement through the shaping bed is increased, and the properties of the glass, in particular the optical properties of the glass, in respect of which it is necessary to increase the time it takes to pass through the shaping bed, whether for shaping the glass sheet or for hardening it, in particular the tempering operation.
The second is that the tilting device situated at the exit from the shaping bed must be regulated precisely on each occasion that the thickness of the glass sheets is modified, since the last two roller/backing roller pairs of the bed need to pinch the rear end of a glass sheet which is in a vertical position in order to tilt it with the aid of a mechanical system onto the conveyor until it is in a horizontal position. Such pinching may mark the glass sheet. Furthermore, the pinching rollers may develop mechanical problems and problems of wear. Such a pinching device is described in EP 346198.